/*

 * Licensed to the Apache Software Foundation (ASF) under one

 * or more contributor license agreements.  See the NOTICE file

 * distributed with this work for additional information

 * regarding copyright ownership.  The ASF licenses this file

 * to you under the Apache License, Version 2.0 (the

 * "License"); you may not use this file except in compliance

 * with the License.  You may obtain a copy of the License at

 *

 *     http://www.apache.org/licenses/LICENSE-2.0

 *

 * Unless required by applicable law or agreed to in writing, software

 * distributed under the License is distributed on an "AS IS" BASIS,

 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.

 * See the License for the specific language governing permissions and

 * limitations under the License.

 */

package com.bff.gaia.unified.sdk.transforms;



import com.bff.gaia.unified.sdk.transforms.display.DisplayData;

import com.bff.gaia.unified.sdk.transforms.display.HasDisplayData;

import com.bff.gaia.unified.sdk.values.TypeDescriptor;

import com.bff.gaia.unified.sdk.coders.Coder;



import javax.annotation.Nullable;

import java.lang.reflect.Method;



/**

 * A {@link ProcessFunction} which is not a <i>functional interface</i>. Concrete subclasses allow

 * us to infer type information, which in turn aids {@link Coder Coder}

 * inference.

 *

 * <p>See {@link SimpleFunction} for providing robust type information where a {@link

 * SerializableFunction} is required.

 */

public abstract class InferableFunction<InputT, OutputT>

    implements ProcessFunction<InputT, OutputT>, HasDisplayData {



  @Nullable

  private final ProcessFunction<InputT, OutputT> fn;



  protected InferableFunction() {

    this.fn = null;

    // A subclass must override apply if using this constructor. Check that via

    // reflection.

    try {

      Method methodThatMustBeOverridden =

          InferableFunction.class.getDeclaredMethod("apply", Object.class);

      Method methodOnSubclass = getClass().getMethod("apply", Object.class);



      if (methodOnSubclass.equals(methodThatMustBeOverridden)) {

        throw new IllegalStateException(

            "Subclass of InferableFunction must override 'apply' method"

                + " or pass a ProcessFunction to the constructor,"

                + " usually via a lambda or method reference.");

      }



    } catch (NoSuchMethodException exc) {

      throw new RuntimeException("Impossible state: missing 'apply' method entirely", exc);

    }

  }



  protected InferableFunction(ProcessFunction<InputT, OutputT> fn) {

    this.fn = fn;

  }



  @Override

  public OutputT apply(InputT input) throws Exception {

    return fn.apply(input);

  }



  public static <InputT, OutputT>

  InferableFunction<InputT, OutputT> fromProcessFunctionWithOutputType(

	  ProcessFunction<InputT, OutputT> fn, TypeDescriptor<OutputT> outputType) {

    return new InferableFunctionWithOutputType<>(fn, outputType);

  }



  /**

   * Returns a {@link TypeDescriptor} capturing what is known statically about the input type of

   * this {@link InferableFunction} instance's most-derived class.

   *

   * <p>See {@link #getOutputTypeDescriptor} for more discussion.

   */

  public TypeDescriptor<InputT> getInputTypeDescriptor() {

    return new TypeDescriptor<InputT>(this) {};

  }



  /**

   * Returns a {@link TypeDescriptor} capturing what is known statically about the output type of

   * this {@link InferableFunction} instance's most-derived class.

   *

   * <p>In the normal case of a concrete {@link InferableFunction} subclass with no generic type

   * parameters of its own (including anonymous inner classes), this will be a complete non-generic

   * type, which is good for choosing a default output {@code Coder<OutputT>} for the output {@code

   * PCollection<OutputT>}.

   */

  public TypeDescriptor<OutputT> getOutputTypeDescriptor() {

    return new TypeDescriptor<OutputT>(this) {};

  }



  /**

   * {@inheritDoc}

   *

   * <p>By default, does not register any display data. Implementors may override this method to

   * provide their own display data.

   */

  @Override

  public void populateDisplayData(DisplayData.Builder builder) {}



  /**

   * A {@link InferableFunction} built from a {@link ProcessFunction}, having a known output type

   * that is explicitly set.

   */

  private static class InferableFunctionWithOutputType<InputT, OutputT>

      extends InferableFunction<InputT, OutputT> {



    private final TypeDescriptor<OutputT> outputType;



    public InferableFunctionWithOutputType(

		ProcessFunction<InputT, OutputT> fn, TypeDescriptor<OutputT> outputType) {

      super(fn);

      this.outputType = outputType;

    }



    @Override

    public TypeDescriptor<OutputT> getOutputTypeDescriptor() {

      return outputType;

    }

  }

}